1. Field of the Invention
The present invention relates to a method and an apparatus for effecting non-contact measurement of a profile or roughness of a surface of a subject, and more particularly to such method and apparatus which uses two light beams for irradiating a minute center spot on the subject surface and an annular spot surrounding the center spot.
2. Discussion of the Prior Art
For measuring the roughness or smoothness or profile of a surface of a subject under examination, without an influence of mechanical vibrations of the subject surface, a non-contact optical measuring system is available. This system usually employs a first light beam for irradiating a relatively small or minute spot on the subject surface, and a second light beam for irradiating a larger spot which is concentric with and has a large diameter than the minute spot. Generally, the larger spot has a diameter of at least 100 .mu.m. The first and second light beams concurrently irradiating the minute and larger spots are reflected by the subject surface, and the reflected light beams interfere with each other so as to produce a beat beam whose beat frequency varies as the subject surface is scanned by the beams. The beat beam is converted into an electrical beat signal, on which a variation in the surface height or level is determined.
The light beam reflected by the minute spot is influenced by both a roughness or height variation and mechanical vibrations of the subject surface. On the other hand, since the surface height variations in the larger spot with a diameter of about 100 .mu.m are averaged to a value substantially equal to zero, the light beam reflected by the larger spot does not include information indicative of the surface height variation, and is influenced by only the mechanical vibrations of the subject surface. As the influences of the mechanical vibrations on the two reflected light beams are almost equal to each other, the beat beam or beat signal produced as a result of interference of the two light beams is substantially free of an influence by the mechanical vibrations of the subject surface, and therefore accurately represents a variation in the surface height or level of the subject.
Conventionally, a double-focus objective lens which has two foci is used to produce the light beams for concurrently irradiating the minute spot and the larger spot on the subject surface. When the surface under examination is placed in one of the two focal planes of the double-focus objective, one of two orthogonally polarized components of a linearly polarized light is reflected from the minute spot, while the other component is out of focus on the subject surface and is reflected from the larger spot. Thus, the minute focused spot and the large defocused spot are formed on the subject surface.
The double-focus objective lens is formed by cutting an expensive doubly-refracting material such as calcite and crystalline quartz, such that the crystal is oriented with respect to the direction of propagation of the incident light. The cut crystal is ground into a desired lens configuration. Accordingly, the double-focus objective lens is considerably expensive.